Excitation Mechanism for the O2 Schumann–Runge System

The Schumann–Runge band system of O₂ involving transitions between the $X^3{\Sigma }_g^{-}$ ground state and the $B^3{\Sigma }_u^{-}$ excited state has been the subject of many investigations which have shown that the radiating state is strongly coupled to the O atoms and that the population mechanism is very fast. In the present study, the emitted radiation was monitored with filter–photomultiplier combinations as a function of time behind incident shock waves in O₂, O₂–Ar, and O₂–Ne mixtures at wavelengths centered at 2300 and 3250 Å. Initial pressures in the shock tube were 1–100 torr and the (equilibrium) temperature range was 2800–5300°K. The radiation, subsequent to a short incubation time, rose monotonically to a plateau level given for each of the two channels by $I_{2300}{\text{\hspace{0.17em}=\hspace{0.17em}4.4\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−36}}[\exp {\text{(−\hspace{0.17em}20.3\hspace{0.17em}×\hspace{0.17em}10}}^3\mathrm{\hspace{0.17em}/\hspace{0.17em}T)](}\mathrm{O}{)}^2$ and $I_{3250}{\text{\hspace{0.17em}=\hspace{0.17em}1.1\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−36}}[\exp {\text{(−\hspace{0.17em}11.3\hspace{0.17em}×\hspace{0.17em}10}}^3\mathrm{\hspace{0.17em}/\hspace{0.17em}T)](}\mathrm{O}{)}^2$ in watts/cubic centimeters·steradians·microns where (O) is the number of O atoms per cubic centimeter. These results are in disagreement with the results of Myers and Bartle both in apparent activation energy and absolute magnitude. In general, these comparisons verified that $\mathrm{I\propto (}\mathrm{O}{)}^2$ during the entire time history—indeed, these data for the oxygen‐rich mixtures yielded a very sensitive measurement of the efficiencies of O₂ and O in dissociating O₂. Various other excitation mechanisms were considered, and it was shown that inverse predissociation is the only one which fits the data and that it must dominate over a very wide range of conditions.